Musculoskeletal injuries of the lower limbs are associated with the strenuous sports and recreational activities. The knee is the most often injured region of the body, with the ACL as the most frequently injured structure of the knee (Lauder et al., Am J Prev. Med., 18: 118-128, 2000). Approximately 80,000 to 250,000 ACL tears occur annually in the U.S. with an estimated cost for the injuries of almost one billion dollars per year (Griffin et al. Am J Sports Med. 34, 1512-32). The highest incidence is in individuals 15 to 25 years old who participate in pivoting sports (Griffin et al., J Am Acad Orthop Surg. 8, 141-150, 2000). Considering that the lower limbs are free to move in the sagittal plane (e.g., knee flexion/extension, ankle dorsi-/plantar flexion), musculoskeletal injuries generally do not occur in sagittal plane movements. On the other hand, joint motion about the minor axes (e.g., knee valgus/varus (synonymous with abduction/adduction), tibial rotation, ankle inversion/eversion and internal/external rotation) is much more limited and musculoskeletal injuries are usually associated with excessive loading/movement about the minor axes (or called off-axes). The ACL is most commonly injured in pivoting and valgus activities that are inherent to sports and high demanding activities, for example.
It is therefore critical to improve neuromuscular control of off-axis motions (e.g., tibial rotation/valgus at the knee) in order to reduce/prevent musculoskeletal injuries and to facilitate post injury rehabilitation. However, existing exercise equipment (e.g., elliptical machine, treadmill, stair climber, stepper, and leg press machine) generally focuses on the sagittal plane movement. Due to the structural limitation, the user simply cannot do the lower extremely control training in the off-axis direction (such as knee valgus/varus, or internal/external rotation, tibial rotation and ankle inversion/eversion). For another solution to the off-axis training, for example, off-axis movement training in a seated posture such as tibial rotation or valgus in isolation is unlikely to be practical and effective since there is no accordingly movement involved in sagittal plane.